1. Field of the Invention
The present invention relates to a spin valve sensor with magnetic and nonmagnetic layers for improving asymmetry and softness of a free layer structure and, more particularly, to a magnetic keeper layer for providing flux closure for a pinned layer structure and a nonmagnetic layer for counterbalancing sense current fields and improving the softness of the free layer structure.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk.
The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. In order to improve the sensitivity of the spin valve sensor a soft magnetic material, such as nickel iron (NiFe), is employed as the free layer. It has been found, however, that when a free layer structure employs a cobalt based layer in addition to the nickel iron (NiFe) free layer that the magnetoresistive coefficient dr/R increases when the cobalt based layer is located between and interfaces the nickel iron (NiFe) free layer and a copper (Cu) spacer layer. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced.
Readback asymmetry is defined as                     V        1            -              V        2                    max      ⁡              (                              V            1                    ⁢                      xe2x80x83                    ⁢          or          ⁢                      xe2x80x83                    ⁢                      V            2                          )              .
For example, +10% readback asymmetry means that the positive readback signal V1 is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in many applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. In these applications +10% readback asymmetry can saturate the free layer in the positive direction and will, of course, reduce the negative readback signal by 10%. An even more subtle problem is that readback asymmetry impacts the magnetic stability of the free layer. Magnetic instability of the free layer means that the applied signal has disturbed the arrangement or multiplied one or more magnetic domains of the free layer. This instability changes the magnetic properties of the free layer which, in turn, changes the readback signal. The magnetic instability of the free layer can be expressed as a percentage increase or decrease in instability of the free layer depending upon the percentage of the increase or decrease of the asymmetry of the readback signal. Standard deviation of the magnetic instability can be calculated from magnetic instability variations corresponding to multiple tests of the free layer at a given readback asymmetry.
There is approximately a 0.2% decrease in standard deviation of the magnetic instability of the free layer for a 1% decrease in readback asymmetry. This relationship is substantially linear which will result in a 2.0% reduction in the standard deviation when the readback asymmetry is reduced from +10% to zero. Magnetic instability of the free layer is greater when the readback asymmetry is positive. Accordingly, the magnetic instability of the free layer is greater when the readback asymmetry is positive than when the readback asymmetry is negative. A positive readback asymmetry can be improved by changing thickness of the magnetic layers and/or changing the sense current, however, a change of one of these parameters can change other parameters making it a trial and error process to reduce the asymmetry. More importantly, however, is that the channel electronics of the disk drive as well as specific thicknesses of the magnetic layers are designed to satisfy other magnetic considerations. Since these values are set it is manifest that there is a need to deal with the many magnetic influences on the free layer of the AP pinned spin valve so that the net value of these influences on the free layer can be reduced to virtually zero thereby reducing the asymmetry to virtually zero.
The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demag field HD from the pinned layer, a sense current field HI from all conductive layers of the spin valve except the free layer, a net image current field HIM from the first and second shield layers. The strongest magnetic force on the free layer structure is the sense current field HI. In an exemplary bottom spin valve sensor where the free layer is closer to the second gap layer than it is to the first gap layer the majority of the conductive layers are below the free layer structure between the free layer structure and the first gap layer. The amount of conductive material in this region is further increased if the pinning layer is metal instead of an oxide, such as nickel oxide (NiO). Accordingly, when the sense current is conducted through the sensor the conductive layers below the free layer structure causes a sense current field on the free layer structure which is minimally counterbalanced by a typical cap layer made of tantalum (Ta) on top of the free layer structure. Accordingly, there is a strong-felt need to counterbalance the strong sense current field exerted by the conductive layers of the spin valve sensor below the free layer structure in a bottom spin valve sensor. Further, the pinned layer structure below the free layer structure in a bottom spin valve sensor exerts a demagnetizing field on the free layer structure which needs to be counterbalanced to improve asymmetry of the spin valve sensor. There is a strong-felt need to counterbalance the sense current and demagnetizing fields and optimize the sense current, the pinning layer structure and the type of material of the pinning layer while still obtaining the desired readback symmetry of the spin valve sensor.
The present invention provides a nonmagnetic conductive layer that interfaces the top of the free layer structure in a bottom spin valve sensor for the purpose of producing a sense current field which counterbalances, to the extent desirable, a sense current field from conductive layers below the free layer structure. In the preferred embodiment the nonmagnetic conductive layer is copper which also improves the softness of the free layer structure so that the magnetic moment of the free layer structure is more responsive in its rotation to magnetic field signals from the rotating magnetic disk. The invention further includes a magnetic keeper layer with the nonmagnetic conductive layer located between the keeper layer and the free layer structure so that the magnetic moment of the keeper layer is isolated from the free layer structure. The keeper layer provides flux closure, to the extent desired, for the demagnetizing field from the pinned layer structure below the free layer structure in the bottom spin valve sensor. In a preferred embodiment the keeper layer is nickel iron (NiFe). In a still further embodiment a second nonmagnetic conductive layer is provided with the keeper layer located between the first and second nonmagnetic conductive layers. The second nonmagnetic conductive layer isolates the keeper layer from a tantalum (Ta) cap layer so as to improve the softness of the keeper layer and provides an additional sense current field for counterbalancing the sense current fields exerted by the conductive layers below the free layer structure.
An object of the present invention is to provide a pair of layers between a free layer structure and a cap layer of a spin valve sensor for improving playback asymmetry as well as improving soft magnetic properties of the free layer structure.
Another object is to provide a copper layer on top of a free layer structure in a spin valve sensor for at least partially counterbalancing sense current fields from conductive layers below the free layer structure and for improving soft magnetic properties of the free layer structure in combination with a magnetic keeper layer on top of the copper layer for providing at least partial flux closure for a pinned layer structure below the free layer structure.
Still another object is to provide a second copper layer on top of the keeper layer in the preceding object for the purpose of improving soft magnetic properties of the keeper layer and providing an additional sense current field for at least partially counterbalancing sense current fields from conductive layers below the free layer structure.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.